Fix for drag start event being incorrectly cached

Since DRAG_END is now being delivered only to views that
returned true from DRAG_STARTED, make sure to clear
mCurrentDragStartEvent when returning false from DRAG_STARTED,
to avoid incorrectly sending DRAG_STARTED events to children
views on visibility changes.

Bug: 27595763

Change-Id: Ic18628b48b474351e2433e871bf545657a6ebf52